Cooperation between Cis and Trans Influences in cis-PtII(PPh3)2

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Inorg. Chem. 2010, 49, 123–135 123 DOI: 10.1021/ic901510m

Cooperation between Cis and Trans Influences in cis-PtII(PPh3)2 Complexes: Structural, Spectroscopic, and Computational Studies Luca Rigamonti,† Alessandra Forni,*,‡ Mario Manassero,*,§ Carlo Manassero,§ and Alessandro Pasini*,† †

Universit a degli Studi di Milano, Dipartimento di Chimica Inorganica, Metallorganica e Analitica “Lamberto Malatesta”, via Venezian 21, 20133 Milano, Italy, ‡ISTM-CNR, via Golgi 19, 20133 Milano, Italy, and § Universit a degli Studi di Milano, Dipartimento di Chimica Strutturale e Stereochimica Inorganica, via Venezian 21, 20133 Milano, Italy Received July 29, 2009

The relevance of cis and trans influences of some anionic ligands X and Y in cis-[PtX2(PPh3)2] and cis-[PtXY(PPh3)2] complexes have been studied by the X-ray crystal structures of several derivatives (X2 = (AcO)2 (3), (NO3)2 (5), Br2 (7), I2 (11); and XY = Cl(AcO) (2), Cl(NO3) (4), and Cl(NO2) (13)), density functional theory (DFT) calculations, and one bond Pt-P coupling constants, 1JPtP. The latter have allowed an evaluation of the relative magnitude of both influences. It is concluded that such influences act in a cooperative way and that the cis influence is not irrelevant when rationalizing the 1JPtP values, as well as the experimental Pt-P bond distances. On the contrary, in the optimized geometries, evaluated through B3LYP/def2-SVP calculations, the cis influence was not observed, except for compounds ClPh (21), Ph2 (22), and, to a lesser extent, Cl(NO2) (13) and (NO2)2 (14). A natural bond order analysis on the optimized structures, however, has shown how the cis influence can be related to the s-character of the Pt hybrid orbital involved in the Pt-P bonds and the net atomic charge on Pt. We have also found that in the X-ray structures of cis-[PtX2(PPh3)2] complexes the two Pt-X and the two Pt-P bond lengths are different each other and are related to the conformation of the phosphine groups, rather than to the crystal packing, since this feature is observed also in the optimized geometries.

Introduction A fundamental aspect of coordination chemistry is the understanding of the influence that a ligand L, in an MLL0n complex, exerts on the properties of the M-L0 bonds, because a knowledge of these influences can give an insight into aspects like stability and reactivity of the complexes under study. In the case of square planar and octahedral complexes, such influences are termed cis and trans influences, depending on which bond is influenced by the “influencing” ligand L.1 The comprehensive importance of these aspects is shown by the fact that, in addition to the multitude of studies performed on transition metal complexes, there are examples also in main group chemistry.2 According to an original definition,1,3 the trans influence of a ligand is related to the extent to which such ligand weakens the bond trans to itself in the ground state of the *To whom correspondence should be addressed. E-mail: alessandro. [email protected] (A.P.), [email protected] (A.F.), mario.manassero@ unimi.it (M.M.). (1) Pidcock, A.; Richards, R. E.; Venanzi, L. M. J. Chem. Soc. (A) 1966, 1707–1710. (2) (a) Ochiai, M.; Sueda, T.; Miyamoto, K.; Kiprof, P.; Zhdankin, V. V. Angew. Chem., Int. Ed. 2006, 45, 8203–8206. (b) Shustorovich, E. M.; Buslaev, Yu. A. Inorg. Chem. 1976, 15, 1142–1147. (3) Venanzi, L. M. Chem. Brit. 1968, 4, 162–167.

r 2009 American Chemical Society

complex. The cis influence can be defined, similarly, as the weakening of a metal-ligand bond induced by a cis ligand. Several techniques have been employed to investigate these influences,4 the most popular being X-ray crystallography2a,4,5 (“structural” trans or cis influence) and NMR spectroscopy. The latter has been used extensively6 when both the metal and the donor atoms are NMR active (such as, in the present case, 195Pt and 31P) by examining the 1J values, (4) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973, 10, 335–422. (5) (a) Kukushkin, V. Yu.; Belsky, V. K.; Konovalov, V. E.; Kirakosyan, G. A.; Konovalov, L. V.; Moiseev, A. I.; Tkachuk, V. M. Inorg. Chim. Acta 1991, 185, 143–154. (b) Manojlovic-Muir, L.; Muir, K. W.; Solomun, T. J. Organomet. Chem. 1977, 142, 265–280. (c) Kapoor, P.; Loevqvist, K.; Oskarsson, Å. J. Mol. Struct. 1998, 470, 39–47. (6) Some examples: (a) Michelin, R. A.; Ros, R. J. Chem. Soc., Dalton Trans. 1989, 1149–1159. (b) Otto, S.; Roodt, A. Inorg. Chim. Acta 2004, 357, 1–10. (c) Roodt, A.; Otto, S.; Steyl, G. Coord. Chem. Rev. 2003, 245, 121–137. (d) Honeychuck, R. V.; Hersh, W. H. Inorg. Chem. 1987, 26, 1826–1828. (e) Arnold, D. P.; Bennett, M. A. Inorg. Chem. 1984, 23, 2117–2124. (f ) Cairns, M. A.; Dixon, K. R.; Rivett, G. A. J. Organomet. Chem. 1979, 171, 373–385. (g) Church, M. J.; Mays, M. J. J. Chem. Soc. (A) 1970, 1938–1941. (h) Appleton, T. G.; Hall, J. R.; Ralph, S. F. Inorg. Chem. 1985, 24, 4685–4693. (7) For instance: (a) Steyn, G. J. J.; Roodt, A.; Poletaeva, I.; Varshavsky, Yu. S. J. Organomet. Chem. 1997, 536-537, 197–205. (b) Mather, G. G.; Pidcock, A.; Rapsey, G. J. N. J. Chem. Soc., Dalton Trans. 1973, 2095–2099. (c) Otto, S.; Roodt, A.; Leipoldt, J. G. S. Afr. J. Chem. 1995, 48, 114–119.

Published on Web 12/01/2009

pubs.acs.org/IC

124 Inorganic Chemistry, Vol. 49, No. 1, 2010 Scheme 1

which can be considered an estimate of the bond strength between these atoms. Correlations between the two studies have also been reported.7 In a previous paper,8 we have used the 1JPtP coupling constants of a series of trans-[PtX2(PPh3)2] and trans-[PtXY(PPh3)2] complexes to investigate the cis influence of some anionic ligands X and Y. We assumed that in these complexes the mutual trans influence of the two phosphines can be considered constant to a good approximation and that the 1 JPtP values of the various complexes should depend, essentially, on the cis influence of X and/or Y. The series of cis influence obtained was the following:8 I > Cl > SePh = SPh = SeEt > NO3 > AcO > NO2 > H > Me > Ph When we moved to the cis-[PtXY(PPh3)2] derivatives, we reasoned that the Pt-P bond strengths must depend on both trans and cis influences. For instance, 1JPtP1 (see Scheme 1) should depend not only on the trans influence of Y, but also on the cis influence of X. The latter has been little studied, and although its contribution is usually believed to be of lower importance,4,9,10 we wondered whether it could be observed, evaluated, and compared with the trans influence. This paper tries and addresses these points. We will discriminate the two contributions analyzing (i) the Pt-P bond distances of the seven X-ray crystal structures we have determined, together with some literature data, (ii) the Pt-P bond distances of the optimized geometries of some derivatives, and (iii) the one bond coupling constants of several compounds. We will also compare the different magnitudes of the cis and trans influences. The nomenclature used in this paper is summarized in Scheme 1. Experimental Section General. Chemicals were reagent grade and used as received. Elemental analyses were performed at the Microanalytical Laboratory of the Universita degli Studi di Milano. 31P{1H} NMR spectra were recorded in CDCl3 on a Bruker Advance DRX 300 at 121 MHz; δP values (ppm) are versus external H3PO4, J coupling constants (Hz) have been found reproducible to (3 Hz. Electrospray ionization (ESI) mass spectra were recorded with a LCQ Advantage Thermofluxional Instrument. Infrared spectra were recorded as KBr disks using a JASCO FTIR 410 spectrophotometer with a 2 cm-1 resolution, and main bands are given in inverse centimeters. cis-[PtCl2(PPh3)2] (1),11 cis-[Pt(AcO)2(PPh3)2] (3),12 and cis-[Pt(NO3)2(PPh3)2] (5)12 were synthesized as described in the literature. Crystals of 3 suitable for X-ray diffraction were obtained by slow (8) Rigamonti, L.; Manassero, C.; Rusconi, M.; Manassero, M.; Pasini, A. Dalton Trans. 2009, 1206–1213. (9) Muenzenberg, R.; Rademacher, P.; Boese, R. J. Mol. Struct. 1998, 444, 77–90. (10) Tau, K. D.; Meek, D. W. Inorg. Chem. 1979, 18, 3574–3580. (11) Hartley, F. R. Organomet. Chem. Rev. A 1970, 6, 119–137. (12) Alesi, M.; Fantasia, S.; Manassero, M.; Pasini, A. Eur. J. Inorg. Chem. 2006, 1429–1435.

Rigamonti et al. evaporation of a diisopropyl ether/chloroform solution, whereas crystals of 5 suitable for X-ray diffraction were obtained by slow diffusion of diisopropyl ether into a chloroform or a dichloromethane solution. All reactions involving silver salts were performed in the dark. Synthetic Procedures: cis-[PtCl(AcO)(PPh3)2] (2). AgAcO (21.1 mg, 0.13 mmol) was added to a solution of 1 (100.1 mg, 0.13 mmol) in CH2Cl2 (10 mL). The mixture was refluxed for 8 h, hot filtered, and the filtrate was evaporated to dryness in vacuo yielding 2 as a white solid (74.2 mg, 70%). Anal. calcd for C38H33ClO2P2Pt 3 CH2Cl2 (899.09): C, 52.10; H, 3.92. Found: C, 52.51; H, 4.09. IR (KBr): 1628, 1370, and 1311 (νCO). δP 3.1 (1P, d, JPP 19, JPtP 3560), 18.3 (1P, d, JPP 19, JPtP 3956). ESI-MS: m/z 719 ([Pt(PPh3)2]þ, 100%), 754 ([PtCl(PPh3)2]þ, 60), 778 ([Pt(AcO)(PPh3)2]þ, 65), 836 ([M þ Na]þ, 25). Crystals suitable for X-ray diffraction were obtained by slow diffusion of diisopropyl ether into a chloroform solution. cis-[PtCl(NO3)(PPh3)2] (4). AgNO3 (21.8 mg, 0.13 mmol) was added to a solution of 1 (94.6 mg, 0.12 mmol) in CH2Cl2 (10 mL). The mixture was refluxed for 8 h, hot filtered, and the filtrate was taken to dryness yielding 4 as a white solid (40.1 mg, 41%). Anal. calcd for C36H30NClO3P2Pt 3 CH2Cl2 (902.05): C, 49.27; H, 3.58; N, 1.55. Found: C, 49.60; H, 3.61; N, 1.55. IR (KBr): 1499 and 1273 (νNO3). δP 2.7 (1P, d, JPP 19, JPtP 3858), 17.6 (1P, d, JPP 19, JPtP 3842). Crystals suitable for X-ray diffraction were obtained by slow diffusion of diisopropyl ether into a chloroform solution. Observed Formation of cis-[PtBrCl(PPh3)2] (6). Solid KBr (174.8 mg, 1.47 mmol) was added to a solution of K2PtCl4 (152.4 mg, 0.37 mmol) in water (5 mL). This solution was added to a solution of PPh3 (192.6 mg, 0.73 mmol) in ethanol (15 mL), and the mixture was stirred at room temperature for 4 h, when a very light yellow solid was recovered by filtration. Its 31P NMR spectrum showed the presence of cis-[PtCl2(PPh3)2] (1), cis-[PtBr2(PPh3)2] (7), and a new series of signals, δP 13.6 (d, JPP 14, JPtP 3665), 15.3 (d, JPP 14, JPtP 3618), assigned to 6, in a 1:1:2 ratio (see the Results and Discussion section). cis-[PtBr2(PPh3)2] (7). Solid KBr (411.7 mg, 3.46 mmol) was added to a solution of K2PtCl4 (199.8 mg, 0.48 mmol) in water (7 mL). The resulting solution was added to a solution of PPh3 (258.2 mg, 0.98 mmol) in ethanol (15 mL), and the mixture was refluxed for 4 h. The very light yellow product was recovered by filtration of the cooled mixture (380.1 mg, 90%). Anal. calcd for C36H30Br2P2Pt (879.47): C, 49.17; H, 3.44. Found: C, 49.40; H, 3.51. δP 14.3 (2P, s, JPtP 3614). ESI-MS: m/z 719 ([Pt(PPh3)2]þ, 80%), 799 ([PtBr(PPh3)2]þ, 100), 902 ([M þ Na]þ, 10). Crystals suitable for X-ray diffraction were obtained by slow diffusion of diisopropyl ether into a chloroform/dichloromethane solution. cis-[PtBr(AcO)(PPh3)2] (8). AgAcO (15.8 mg, 0.095 mmol) was added to a solution of 7 (80.3 mg, 0.091 mmol) in CH2Cl2 (10 mL). The mixture was stirred at room temperature for 8 h, filtered hot, and the filtrate was concentrated and precipitated with diisopropyl ether, yielding a very light yellow solid (65.5 mg, 80%). Anal. calcd for C38H33BrO2P2Pt 3 2H2O (894.64): C, 51.02; H, 4.25. Found: C, 51.03; H, 3.95. ESI-HRMS: m/z Calcd for [M þ Na]þ: 881.06748. Found: 881.06687. δP 2.0 (1P, d, JPP 17, JPtP 3538), 18.8 (1P, d, JPP 17, JPtP 3907). cis-[PtBr(NO3)(PPh3)2] (9). AgNO3 (22.1 mg, 0.13 mmol) was added to a solution of 7 (111.0 mg, 0.13 mmol) in CH2Cl2 (15 mL). The mixture was refluxed for 4 h, filtered hot, and the filtrate was concentrated and precipitated with diisopropyl ether, yielding a very light yellow solid. The crude product was recrystallized from CH2Cl2-n-hexane (30.1 mg, 27%). Anal. calcd for C36H30BrNO3P2Pt (861.568): C, 50.19; H, 3.51; N, 1.63. Found: C, 50.34; H, 3.48; N, 1.76. δP 1.5 (1P, d, JPP 17, JPtP 3826), 17.8 (1P, d, JPP 17, JPtP 3790). cis-[PtClI(PPh3)2] (10). A solution of KI (672.2 mg, 4.48 mmol) in H2O (5 mL) and EtOH (5 mL) was added to a white suspension of 1 (86.5 mg, 0.11 mmol) in CHCl3 (5 mL) and

Article acetone (5 mL). The yellow mixture was left under reflux for 1 h, and then the yellow solid was filtered, washed with H2O, EtOH, and diisopropyl ether, and dried. The crude product was recrystallized from CH2Cl2-diisopropyl ether, yielding 10 as light yellow polycrystalline solid (42.1 mg, 43%). Anal. calcd for C36H30ClIP2Pt (882.02): C, 49.02; H, 3.43. Found: C, 49.29; H, 3.32. δP 11.6 (1P, d, JPP 12, JPtP 3683), 13.7 (1P, d, JPP 12, JPtP 3420). [PtI2(cod)]. Solid KI (833.7 mg, 5.02 mmol) was added to a solution of K2PtCl4 (327.6 mg, 0.79 mmol) in water (10 mL). After 10 min, acetic acid (10 mL) and cod (390 μL, 3.17 mmol) were added to the dark solution. The mixture was left at 80 °C for 2 h. After cooling, the product was recovered as a yellow solid by filtration, washed with water, ethanol, and diisopropyl ether, and dried in vacuo (395.2 mg, 90%). Anal. calcd for C8H12I2Pt (557.07): C, 17.25; H, 2.17. Found: C, 17.40; H, 2.36. IR (KBr): 1499, 1422, and 1340 (νCC). cis-[PtI2(PPh3)2] (11). A solution of PPh3 (107.8 mg, 0.41 mmol) in CH2Cl2 (5 mL) was added dropwise to a solution of [PtI2(cod)] (119.6 mg, 0.21 mmol) in CH2Cl2 (5 mL) and the mixture was stirred at room temperature for 5 min. The resulting pale yellow solid was filtered, washed with CH2Cl2 and diisopropyl ether, and dried in vacuo (183.9 mg, 87%). Anal. calcd for C36H30I2P2Pt 3 3H2O (1027.51): C, 42.08; H, 3.53. Found: C, 42.11; H, 3.63. 31P NMR showed that this solid consisted of a 1:2 mixture of the cis and trans isomers: δP 12.6 (s, JPtP 2493) trans-[PtI2(PPh3)2], 11.9 (s, JPtP 3455) cis-[PtI2(PPh3)2] (11). Refluxing this mixture in ethanol (15 mL) in the presence of an excess of PPh3 gave complete isomerization to trans-[PtI2(PPh3)2] (NMR evidence). Slow crystallization from chloroform-diisopropyl ether gave deep orange and yellow crystals. The former consisted of trans-[PtI2(PPh3)2], whose structure is already published.13 The yellow crystals were found to be suitable for X-ray crystallography and consisted of 11. cis-[PtI(NO3)(PPh3)2] (12). KI (13.3 mg, 0.080 mmol) was added to a solution of 5 (130.3 mg, 0.15 mmol) in CH2Cl2 (5 mL) and CHCl3 (5 mL). The mixture was refluxed for 4 h and then filtered hot. The filtrate was taken to dryness yielding a yellow-orange solid. The crude product was recrystallized twice from dichloromethane-diisopropyl ether, yielding 12 as a yellow powder (45.6 mg, 30%). Anal. calcd for C36H30INO3P2Pt 3 CHCl3 (1027.95): C, 43.23; H, 3.04; N, 1.36. Found: C, 42.98; H, 3.33; N, 1.32. IR (KBr): 1497 and 1272 (νNO3). δP -0.5 (1P, d, JPP 14, JPtP 3819), 15.5 (1P, d, JPP 14, JPtP 3596). cis-[PtCl(NO2)(PPh3)2] (13). AgNO2 (87.8 mg, 0.57 mmol) was added to a solution of 1 (100.3 mg, 0.13 mmol) in CHCl3: CH2Cl2 1:1 (10 mL). The mixture was refluxed for 3 days, hot filtered, and the filtrate was evaporated to dryness yielding 13 as a white solid (105.2 mg, 99%). Anal. calcd for C36H30NClO2P2Pt (801.12): C, 53.97; H, 3.77; N, 1.75. Found: C, 53.56; H, 4.42; N, 1.86. IR (KBr): 1410 and 1335 (νNO2). δP -0.5 (1P, d, JPP 20, JPtP 2883), 12.2 (1P, d, JPP 20, JPtP 3949). ESI-MS: m/z 719 ([Pt(PPh3)2]þ, 50%), 754 ([PtCl(PPh3)2]þ, 100), 765 ([Pt(NO2)(PPh3)2]þ, 90), 824 ([M þ Na]þ, 15), 1625 ([2 M þ Na]þ, 50). Crystals suitable for X-ray diffraction were obtained by slow diffusion of diisopropyl ether into a chloroform solution. cis-[Pt(NO2)2(PPh3)2] (14). The reaction was performed under an argon atmosphere in a Schlenk tube. Solid AgNO2 (230.8 mg, 1.50 mmol) was added to a solution of 1 (94.5 mg, 0.12 mmol) in dried CHCl3 (7 mL), and the mixture was left under reflux for 15 h. The mixture was filtered hot and the filtrate was taken almost to dryness, yielding a very light yellow solid, filtrated and washed with diisopropyl ether (85.6 mg, 88%). Anal. calcd for C36H30N2O4P2Pt (811.67): C, 53.27; H, (13) Boag, N. M.; Mohan Rao, K.; Terrill, N. J. Acta Cryst. Sect. C 1991, C47, 1064–1065.

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3.73; N, 3.45. Found: C, 53.36; H, 3.65; N, 3.56. IR (KBr): 1412 and 1336 (νNO2). δP -1.3 (2P, s, JPtP 3148). X-ray Data Collections and Structure Determinations. Crystal data are summarized in Table 1. The diffraction experiments were carried out on a Bruker APEX II CCD area-detector diffractometer, at 150 K for 7 and 13, and at 296 K for 2, 3 3 0.25H2O, 4 3 CHCl3, 5 3 CH2Cl2, 5 3 CHCl3, and 11 3 H2O, using Mo KR radiation (λ = 0.71073 A˚) with a graphite crystal monochromator in the incident beam. No crystal decay was observed, so that no time-decay correction was needed. The collected frames were processed with the software SAINT,14 and an empirical absorption correction was applied (SADABS)15 to the collected reflections. The calculations were performed using the Personal Structure Determination Package16 and the physical constants tabulated therein.17 The structures were solved by direct methods (SHELXS)18 and refined by full-matrix least-squares using all reflections and minimizing the function Σw(Fo2 - kFc2)2 (refinement on F2). In 4 3 CHCl3, the solvent molecule is disordered: the carbon atom (C1) and the first Cl atom (Cl2) are ordered, whereas the second Cl atom is split into two peaks (Cl3 and Cl4) having occupancies of 0.50 each, and the third Cl atom is split into four peaks (Cl5, Cl6, Cl7, and Cl8) having occupancies of 0.25 each. In spite of this, all the nonhydrogen atoms of this molecule could be refined with anisotropic thermal factors, whereas the hydrogen atom was obviously ignored. In 3 3 0.25H2O, the water oxygen atom has an occupancy factor of 0.50 but could be refined with an anisotropic thermal factor. Surprisingly, its two hydrogen atoms were detected in the final Fourier maps and included in the structure factor calculations with occupancies 0.50 each, and not refined. In 11 3 H2O, the oxygen atom of the solvent molecule has an occupancy factor of 1.00, but its hydrogen atoms were not detected in the final Fourier maps and were ignored. In 5 3 CH2Cl2 and 5 3 CHCl3, the solvent molecules are ordered and were treated normally. In 2 and 3 3 0.25H2O, the hydrogen atoms of the CH3 groups belonging to the acetato ligands were detected in the final Fourier maps and not refined. All the other hydrogen atoms were placed in their ideal positions (C-H = 0.97 A˚), with the thermal parameter U being 1.10 times that of the atom to which they are attached, and not refined. All the non-hydrogen atoms of the eight compounds were refined with anisotropic thermal parameters. For chiral 2, full refinement of the correct structure enantiomorph led to R2=0.028 and R2w= 0.043, full refinement of the wrong one led to R2 = 0.096 and R2w=0.163. For chiral 3 3 0.25H2O, full refinement of the correct (14) SAINT Reference manual; Siemens Energy and Automation: Madison, W1, 1994-1996. (15) Sheldrick, G. M. SADABS, Empirical Absorption Correction Program; University of G€ottingen, G€ottingen, Germany, 1997. (16) Frenz, B. A. Comput. Phys. 1988, 2, 42–48. (17) Crystallographic Computing 5; Oxford University Press: Oxford, U.K., 1991; Chapter 11, p 126. (18) Sheldrick, G. M. SHELXS 86. Program for the solution of crystal structures; University of G€ottingen, G€ottingen, Germany, 1985. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.04; Gaussian, Inc.: Pittsburgh, PA, 2003.

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Table 1. Crystallographic Data compound

2

3 3 0.25H2O

4 3 CHCl3

5 3 CH2Cl2

formula M crystal system space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg U/A˚3 Z F(000) Dc/g 3 cm-3 crystal dimensions/mm μ (Mo KR)/cm-1 minimum and maximum transmission factors T/K λ (Mo KR) scan mode frame width/deg time per frame/s no. of frames detector-sample distance/cm θ range reciprocal space explored no. of reflections (total; independent) Rint final R2 and R2w indicesa (F2, all reflections) conventional R1 index (I > σ(I)) reflections with I > 2σ(I) no. of variables goodness of fitb

C38H33ClO2P2Pt 814.18 orthorhombic P212121 10.1657(6) 17.6477(10) 18.5574(10) 90 90 90 3329.2(3) 4 1608 1.624 0.25  0.35  0.45 44.65 0.677-1.000 296 0.71073 ω 0.50 15 3960 6.00 3.00-29.00 full sphere 112229; 8960 0.0286 0.028, 0.043 0.017 8500 397 0.988

C160H146O17P8Pt4 3369.09 orthorhombic P212121 10.4372(7) 18.6546(12) 34.9948(22) 90 90 90 6813.6(9) 2 3348 1.642 0.05  0.45  0.50 42.94 0.556-1.000 296 0.71073 ω 0.50 10 2580 6.50 3.00-28.00 full sphere 131789; 16557 0.0508 0.044, 0.063 0.028 15201 856 1.007

C37H31Cl4NO3P2Pt 936.51 triclinic P1 11.1572(5) 11.6921(5) 14.6755(7) 92.144(1) 96.166(1) 106.110(1) 1824.1(1) 2 920 1.705 0.12  0.20  0.25 43.04 0.594-1.000 296 0.71073 ω 0.50 20 3013 5.00 3.00-29.00 full sphere 60430; 11538 0.0246 0.031, 0.048 0.022 9926 469 1.012

C37H32Cl2N2O6P2Pt 928.62 monoclinic P21/n 11.9715(7) 20.0334(11) 15.4580(9) 90 95.190(1) 90 3692.1(4) 4 1832 1.671 0.32  0.34  0.37 41.16 0.822-1.000 296 0.71073 ω 0.50 20 3300 6.00 3.00-28.00 full sphere 104889; 9948 0.0296 0.031, 0.047 0.020 7911 451 1.039

Compound

5 3 CHCl3

7

11 3 H2O

13

formula M crystal system space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg U/A˚3 Z F(000) Dc/g 3 cm-3 crystal dimensions/mm μ (Mo KR)/cm-1 minimum and maximum transmission factors T/K λ (Mo KR) scan mode frame width/deg time per frame/s no. of frames detector-sample distance/cm θ range reciprocal space explored no. of reflections (total; independent) Rint final R2 and R2w indicesa (F2, all reflections) conventional R1 index (I > σ(I)) reflections with I > 2σ(I) no. of variables goodness of fitb

C37H31Cl3N2O6P2Pt 963.07 monoclinic P21/n 12.0353(6) 19.9799(10) 15.8339(8) 90 95.330(1) 90 3791.0(3) 4 1896 1.687 0.03  0.03  0.25 40.81 0.571-1.000 296 0.71073 ω 0.50 30 2220 5.00 3.00 - 29.00 full sphere 99490; 13130 0.0719 0.050, 0.057 0.034 8373 460 0.976

C36H30Br2P2Pt 879.50 monoclinic P21/c 32.6290(21) 9.6539(6) 19.8452(13) 90 93.810(1) 90 6237.4(8) 8 3392 1.873 0.08  0.35  0.45 72.10 0.275-1.000 150 0.71073 ω 0.50 20 2880 6.00 3.00 - 26.00 full sphere 113116; 13475 0.0567 0.069, 0.116 0.048 8896 740 1.002

C36H30I2OP2Pt 989.49 triclinic P1 9.8799(5) 10.5936(6) 17.8664(9) 86.690(1) 78.110(1) 69.880(1) 1718.0(2) 2 936 1.913 0.20  0.25  0.45 60.22 0.496-1.000 296 0.71073 ω 0.50 20 2400 5.00 3.00 - 29.00 full sphere 48198; 11669 0.0286 0.059, 0.096 0.034 7302 370 0.986

C36H30ClNO2P2Pt 801.14 monoclinic P21/c 9.5838(2) 17.2369(3) 19.3496(3) 90 99.869(1) 90 3149.2(1) 4 1576 1.690 0.21  0.23  0.48 47.20 0.499-1.000 150 0.71073 ω 0.40 10 1320 6.00 3.00 - 26.00 full sphere 25153; 7362 0.0550 0.063, 0.086 0.038 4577 388 0.974

a R2 = [Σ(|Fo2 - kFc2|/ΣFo2], R2w = [Σw(Fo2 - kFc2)2/Σw(Fo2)2]1/2. b [Σw(Fo2 - kFc2)2/(No - Nv)]1/2, where w = 4Fo2/σ(Fo2)2, σ(Fo2) = [σ2(Fo2) þ (pFo2)2]1/2, No is the number of observations, Nv is the number of variables, p = 0.02 for 4 3 CHCl3, 5 3 CH2Cl2, 5 3 CHCl3, and 11 3 H2O, and p = 0.03 for 2, 3 3 0.25H2O, 7, and 13.

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Preparation and Characterization. The majority of the compounds were prepared by metathesis reaction, treating cis-[PtCl2(PPh3)2] (1) with the appropriate silver or potassium salt. cis-[PtI2(PPh3)2] (11) was prepared by reaction of [PtI2(cod)] (cod = cyclooctadiene) with PPh3. This reaction, however, gave a mixture of cis/trans derivatives. Also the reported preparation of 11 by treatment of 1 with NaI26 gave, in our hands, a substantial amount of the trans isomer. Pure samples of the cis isomer were obtained by careful crystallization of these mixtures. Interestingly, refluxing an ethanol suspension of these mixtures with excess PPh3 gave pure trans-[PtI2(PPh3)2], in contrast with the behavior of the cis/trans dichloro complexes, for which a similar treatment gives rise to complete isomerization to 1.27 The synthesis of cis-[PtBr2(PPh3)2] (7) was carried out following the same synthetic procedure of 1,11 from K2[PtBr4], prepared in situ by the addition of a large excess of KBr added to K2[PtCl4]. If the synthesis of 7 was performed using lower amounts of KBr, we detected an intermediate. Under these conditions, the 31P NMR spectra of the crude products showed the presence of the singlets of 1 and 7 and two doublets of the new compound. The concentrations of 1 and the latter diminished, increasing the amount of KBr, and eventually disappeared when a large excess was used, giving

7 analytically pure. We interpreted these facts as the observation of the stepwise substitution of Cl by Br, and consequently assigned the two doublets to cis-[PtBrCl(PPh3)2] (6) unambiguously. Attempts to isolate it failed, because of the very similar solubility of 1, 6, and 7 (in the Experimental Section we reported the conditions which give the highest concentration of 6 in the mixture). cis-[PtBr(NO3)(PPh3)2] (9), cis-[PtClI(PPh3)2] (10) and cis-[PtI(NO3)(PPh3)2] (12) were obtained pure only after careful recrystallization, but in rather low yields. The synthesis of cis-[Pt(NO2)2(PPh3)2] (14) was found to be rather “tricky”, since this compound was often impure of the oxidation product cis-[Pt(NO3)2(PPh3)2] (5); only performing the reaction under a strict argon atmosphere we managed to obtain 14 analytically pure. The cis configuration of compounds 2, 3, 4, 5, 7, 11, and 13 was unambiguously assigned by X-ray structural determinations (see below). The cis geometry of the mixed cis-[PtXY(PPh3)2] complexes was confirmed by the presence, in their 31P NMR spectra, of two doublets, due to the nonequivalent P atoms. An IR criterion8,26,28 was also used: an intense band at around 550 cm-1, probably a PC3 overtone, has been reported to be typical of cis-Pt(PPh3)2 complexes, it being absent, or very weak, in the trans derivatives. This criterion was found to hold each time we could compare it with X-ray structures or 31P NMR evidence. X-ray Structures. We have performed the X-ray structural determinations of compounds 2, 3 3 0.25H2O, 4 3 CHCl3, 5 3 CH2Cl2, 5 3 CHCl3, 7, 11 3 H2O, and 13. ORTEP views of compounds 3, 11, and 13 are presented in Figures 1-3; pictures of the other compounds can be found in the Supporting Information. Relevant bond lengths and angles, together with those of compound 1,29 are collected in Tables 2-4. All complexes present a square planar coordination of the Pt atom, with minor distortions. In 3 the two acetato and in 5 the two nitrato groups are almost perpendicular to the coordination plane and in an anticonfiguration with respect to it, giving rise to chirality.30 In 3 3 0.25H2O the elementary cell contains both enantiomeric arrangements. Bond lengths and angles are within the normal ranges, the Pt-P bonds are shorter while the Pt-Cl and Pt-O (acetato and nitrato) are longer than those found in the corresponding trans isomers.8,13,31,32 A careful inspection of the structures reveals an interesting feature that to our knowledge has never been described previously. In fact there is a relationship

(20) (a) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785– 789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (21) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297– 3305. (22) (a) Feller, D. J. Comput. Chem. 1996, 17, 1571–1586. (b) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. J. Chem. Inf. Model. 2007, 47, 1045–1052. (23) Peterson, K. A.; Figgen, D.; Goll, E.; Stoll, H.; Dolg, M. J. Chem. Phys. 2003, 119, 11113–11123. (24) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123–141. (25) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899– 926. (26) Mastin, S. H. Inorg. Chem. 1974, 13, 1003–1005. (27) Anderson, G. K.; Cross, R. J. Chem. Soc. Rev. 1980, 9, 185–215.

(28) (a) Brune, H. A.; Ertl, J.; Grafl, D.; Schmidtberg, G. Chem. Ber. 1982, 115, 1141–1153. (b) Brune, H. A.; Ertl, J. Liebigs Ann. Chem. 1980, 928– 937. (29) Fun, H.-K.; Chantrapromma, S.; Liu, Y.-C.; Chen, Z.-F.; Liang, H. Acta Cryst. Sect. E 2006, E62, m1252–m1254. (30) For a discussion on the chirality of square planar complexes see, for instance: (a) Gullotti, M.; Pacchioni, G.; Pasini, A.; Ugo, R. Inorg, Chem. 1982, 21, 2006–2014. (b) Pasini, A.; De Giacomo, L. Inorg. Chim. Acta 1996, 248, 225–230. (c) Ano, S. O.; Intini, F. P.; Natile, G.; Marzilli, L. G. Inorg. Chem. 1999, 38, 2989–2999. (d) Chifotides, H. T.; Dunbar, K. R. Chem. Eur. J. 2008, 14, 9902–9913. (e) Ranaldo, R.; Margiotta, N.; Intini, F. P.; Pacifico, C.; Natile, G. Inorg. Chem. 2008, 47, 2820–2830. (31) Basato, M.; Biffis, A.; Martinati, G.; Tubaro, C.; Venzo, A.; Ganis, P.; Benetollo, F. Inorg. Chim. Acta 2003, 355, 399–403. (32) Johanson, M. H.; Otto, S. Acta Cryst. Sect. C 2000, C56, e12–e15.

structure enantiomorph led to R2 =0.044 and R2w =0.063, full refinement of the wrong one led to R2=0.096 and R2w=0.153. In the final Fourier maps, the maximum residuals were 1.94(13) e 3 A˚-3 at 0.73 A˚ from Pt, 3.01(26) e 3 A˚-3 at 0.83 A˚ from Pt, 1.71(18) e 3 A˚-3 at 0.63 A˚ from Pt, 1.18(20) e 3 A˚-3 at 0.08 A˚ from Cl2, 1.44(44) e 3 A˚-3 at 1.03 A˚ from P1, 2.63(78) e 3 A˚-3 at 0.62 A˚ from Pt, 2.42(73) e 3 A˚-3 at 0.55 A˚ from Pt, and 2.41(50) e 3 A˚-3 at 0.78 A˚ from Pt, for 2, 3 3 0.25H2O, 4 3 CHCl3, 5 3 CH2Cl2, 5 3 CHCl3, 7, 11 3 H2O, and 13, respectively. Minimum peaks (holes), in the same order, were -1.00(13), -2.63(26), -1.33(18), -0.96(20), -1.28(44), -3.35(78), -2.35(73), and -1.94(50) e 3 A˚-3, respectively. CCDC 741800-741807 contain the supplementary crystallographic data for this paper. Computational Details. Calculations were performed with the Gaussian 03 program package.19 All geometries were fully optimized using the B3LYP functional20 and the def2-SVP split valence basis set with polarization functions on all atoms.21,22 In particular, an extra set of f functions with exponent 0.66813 has been used for Pt. I and Pt have been treated as a 25- and an 18-electron system, respectively, with relativistic effective core potentials (ECP’s) taken from the literature.23,24 The natural bond orbital (NBO) analysis25 was used for evaluating the s character of the hybrid orbitals involved in the Pt-P bonds in the different complexes.

Results and Discussion

128 Inorganic Chemistry, Vol. 49, No. 1, 2010

Figure 1. ORTEP view of one independent molecule of cis-[Pt(AcO)2(PPh3)2] (3 3 0.25H2O). Ellipsoids are drawn at the 30% probability; hydrogen atoms and the water molecule are omitted for clarity.

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Figure 3. ORTEP view of cis-[PtCl(NO2)(PPh3)2] (13). Ellipsoids are drawn at the 30% probability; hydrogen atoms are omitted for clarity. Table 2. Main Distances (A˚) and Angles (deg) of the X2 Compounds 1, 7, and 11 3 H2Oa 1b, X = Cl

Pt-P1 Pt-P2 Pt-X1 Pt-X2 P1-Pt-P2 P2-Pt-X1 X2-Pt-P1 X1-Pt-X2 P1-Pt-X1 P2-Pt-X2 Ac Bd

Figure 2. ORTEP view of cis-[PtI2(PPh3)2] (11 3 H2O). Ellipsoids are drawn at the 30% probability; hydrogen atoms and the water molecule are omitted for clarity.

between the conformations of the phosphine groups and Pt-X/Y bond lengths. In the case of the X2 complexes (with the exception of molecule 2 of 1 and molecule 2 of 7), the two phosphine groups display different conformations: the ipso carbon atom C (in bold in Scheme 1) of phosphine A is almost eclipsed to the X ligand cis to it: the X-Pt-P-C torsion angles vary from 2 to 14° (Tables 2-4); the lowest X-Pt-P-C torsion angles of phosphine B are much larger (34-54°). In the PtX2 complexes which present this feature, the shorter Pt-P bond is that of phosphine A (i.e., the P atom involved in such eclipsed conformation), while the longer Pt-X bond is trans to such phosphine. In the structures of the three PtClX complexes 2, 4, and 13, phosphine A (the one with the eclipsed C atom) is always cis to Y. We think that such adopted molecular conformation allows to optimizing the π-π interaction between two phenyl rings of the two phosphines.

7, X = Br

molecule 1 molecule 2 molecule 1 molecule 2

11 3 H2O, X=I

2.252(1) 2.271(1) 2.363(1) 2.329(1) 99.12(3) 83.73(3) 89.82(3) 87.56(3) 175.57(3) 170.41(3) 12.6(1) -59.5(1)

2.277(1) 2.292(1) 2.656(1) 2.634(1) 97.83(5) 85.17(4) 90.64(3) 86.78(1) 174.87(4) 170.06(3) -14.3(2) 43.7(2)

2.271(1) 2.261(1) 2.351(1) 2.348(1) 99.93(4) 87.35(4) 86.25(4) 87.73(4) 167.58(4) 171.13(4) 34.1(1) 47.8(1)

2.264(2) 2.285(2) 2.475(1) 2.451(1) 98.95(6) 84.02(5) 90.01(5) 87.41(3) 175.00(5) 169.77(5) 14.7(2) 60.7(2)

2.278(2) 2.294(2) 2.485(1) 2.473(1) 99.19(7) 86.69(5) 87.71(5) 87.36(3) 171.54(6) 169.41(7) 34.1(3) 46.7(2)

a P1 trans to X1 for all compounds. b Adapted from ref 29. c A = lowest X2-Pt-P-C torsion angle of phosphine A. d B = lowest X1-Pt-P-C torsion angle of phosphine B.

Strucural Cis and Trans Influences. In order to study the “structural” cis and trans influences of ligands X and/or Y, we must compare the Pt-P bond distances of the various complexes. To render the discussion easier, this is presented in a table format (Table 5), where we have collected the Pt-P bond distances of the ClX complexes, the Pt-P mean values of the X2 complexes, and three Pt-P mean values taken from the literature.29,33,34 Although the individual bond lengths display significant differences, we decided to use the mean values for X2 derivatives in order to compare these results with the spectroscopic data in solution (one NMR signal); the different bond lengths in the solid state are due to crystal (33) Yahav, A.; Goldberg, I.; Vigalok, A. Inorg. Chem. 2005, 44, 1547– 1553. (34) (a) Hannu-Kuure, M. S.; Komulainen, J.; Oilunkaniemi, R.; Laitinen, R. S.; Suontamo, R.; Ahlgren, M. J. Organomet. Chem. 2003, 666, 111–120. (b) Hannu, M. S.; Oilunkaniemi, R.; Laitinen, R. S.; Ahlgren, M. Inorg. Chem. Commun. 2000, 3, 397–399.

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Table 3. Main Distances (A˚) and Angles (deg) of the X2 Compounds 3 3 0.25H2O, 5 3 CH2Cl2, and 5 3 CHCl3a 3 3 0.25H2O, X = AcO

Pt-P1 Pt-P2 Pt-O1 Pt-O3/O4 P1-Pt-P2 P2-Pt-O1 O3/O4-Pt-P1 O1-Pt-O3/O4 P1-Pt-O1 P2-Pt-O3/O4 Ab Bc

5, X = NO3

molecule 1

molecule 2

5 3 CH2Cl2

5 3 CHCl3

2.240(1) 2.243(1) 2.070(3) 2.059(3) 97.90(4) 87.41(8) 89.16(8) 85.44(11) 174.07(9) 172.64(8) 7.30(17) 33.41(18)

2.233(1) 2.250(1) 2.079(3) 2.050(3) 96.44(4) 86.96(8) 89.48(8) 87.16(11) 173.97(9) 174.08(8) -49.06(16) -49.06(16)

2.239(1) 2.244(1) 2.088(2) 2.092(2) 97.33(2) 86.58(4) 91.17(5) 84.96(6) 174.43(4) 171.49(5) 4.58(11) -47.70(9)

2.237(1) 2.247(1) 2.092(2) 2.097(2) 96.96(2) 87.41(5) 91.08(6) 84.56(7) 174.65(5) 171.96(6) 1.37(9) -49.76(10)

a P1 trans to O1 for all compounds, P2 trans to O3 for 3 and to O4 for 5. b A = lowest O3/O4-Pt-P1-C torsion angle of phosphine A. c B = lowest O1-Pt-P2-C torsion angle of phosphine B.

Table 4. Main Distances (A˚) and Angles (deg) of ClX Compounds 2, 4 3 CHCl3, and 13a

Pt-P1 Pt-P2 Pt-Cl Pt-O1/N P1-Pt-P2 P2-Pt-Cl O1/N-Pt-P1 Cl-Pt-O1/N P1-Pt-Cl P2-Pt-O1/N Ab Bc

2, X = AcO

4 3 CHCl3, X = NO3

13, X = NO2

2.244(1) 2.249(1) 2.345(1) 2.070(2) 98.77(2) 87.58(2) 89.32(5) 84.44(5) 172.36(2) 171.80(5) -2.18(18) 54.50(9)

2.252(1) 2.243(1) 2.362(1) 2.109(1) 98.78(2) 87.30(2) 89.73(4) 84.28(4) 173.65(2) 171.15(4) -14.89(8) -51.54(7)

2.260(1) 2.284(1) 2.335(1) 2.110(4) 97.39(5) 88.77(5) 90.28(11) 83.55(11) 173.83(5) 172.12(11) 7.48(21) -42.70(18)

a P1 trans to Cl for all compounds; P2 trans to O1 for 2 and 4 and to N for 13. b A = lowest O1/N-Pt-P1-C torsion angle of phosphine A. c B = lowest Cl-Pt-P2-C torsion angle of phosphine B.

packing effects as well as the phoshine conformations, as described above, while in solution a mean distance with a symmetric geometry is expected. Appropriate comments and the deductions drawn from such values are inserted in the table. To begin with, let us consider only the mean Pt-P bond lengths of the X2 complexes. Following the usually accepted hypothesis4,9,10 that the cis influence has a lower relevance, we obtain the “structural” trans influence series PhSe > I > Br > Cl > NO3 ∼ AcO > F. The trans influence can also be observed, for the few data available, from the Pt-P2 bond distances of the mixed ClX complexes, which decreases in the order 1 > 2 > 4 ∼ 13, giving another trans influence series Cl > AcO > NO3 ∼ NO2. The difference of the position of AcO and NO3 between the two series demonstrates that the cis contribution cannot be neglected. The cis influence is in fact evident if we look at the Pt-P1 bond lengths of the mixed ClX derivatives, which decrease in the order 1 > 13 > 4 > 2, giving the cis influence order Cl > NO2 > NO3 > AcO.35 (35) Comparison with the sequences obtained in the previous paper8 holds only for NO2 > NO3 > AcO; the position of Cl changes according to the class of complexes considered, suggesting that the actual influences of a ligand depend on its environment, and this will be discussed later.

129

Optimized Geometries. We have performed gas-phase theoretical calculations on the investigated complexes in order to draw further insight on the structural trans and cis influence series. Selected results of geometry optimization of cis-[PtX2(PPh3)2] and cis-[PtClX(PPh3)2] complexes by B3LYP/def2-SVP calculations are reported in Tables 6 and 7, respectively. Comparison with the available corresponding experimental results indicates that the adopted method reproduces accurately (within 0.02 A˚) the Pt-O distances, in particular for cis-[Pt(NO3)2(PPh3)2] (5). All other interatomic distances involving Pt are overestimated by 0.02-0.08 A˚, a typical lengthening of metal-ligand bonds generally obtained by gradientcorrected density functional theory (DFT) methods.36 An exception to such lengthening is for cis-[PtCl(NO2)(PPh3)2] (13), in which Pt-P2 is 0.112 A˚ longer than the experimental value and Pt-N is underestimated by 0.034 A˚. Geometry optimization of the X2 complexes allows to reproduce the experimental finding concerning the relative orientation of the phenyl rings and the consequent deviation from symmetry of the coordination geometry of the metal atom. In all cases, except for X=Ph, we have obtained molecular conformations where atom X2 is nearly eclipsed to the ipso C atom bonded to P1 (phosphine A of Scheme 1), while X1 forms much larger torsion angles with all the ipso C atoms bonded to P2 of phosphine B (see Table 6, torsions A and B, respectively). In agreement with the experimental structures, the Pt-P1 bonds are systematically shorter than the Pt-P2 bonds. The exception is cis-[Pt(Ph)2(PPh3)2] (22), where interference of the phenyl ligands with the PPh3 groups stabilizes a conformation in which both Ph ligands are located nearly midway the two positions assumed by the substituents in the other complexes. By looking at the Pt-P1 (or Pt-P2) distances of X2 complexes, we see that the structural trans influence series reported above is confirmed by our calculations, with the order Ph > NO2 > I > Br > Cl > AcO g NO3. As for the ClX complexes, optimization has been performed starting from cis-[PtCl2(PPh3)2] (1). Because of the observed nonequivalence of the two Pt-Cl bond distances, we have substituted alternatively the Cl positions with an X group. The conformation with X cis to phosphine A was found more stable by up to 3.9 kcal/mol, and this is in fact the only one observed in the available experimental structures of the mixed complexes. Examination of Table 7 reveals that in such complexes the Pt-P1 bonds (trans to Cl) are virtually unvaried with respect to the Pt-P1 bond of the Cl2 complex (2.320 A˚), suggesting that the presence of an X ligand cis to P1 has no substantial influence (the exceptions are for X = Ph, and, in much lesser extent, NO2). The Pt-P2 distances (trans to X) are also almost unchanged with respect to the corresponding Pt-P2 distances in the X2 complexes, excepted for Ph, highlighting the nonobservable structural cis influence. Then, the order of the structural trans influence obtained for the X2 complexes is confirmed also (36) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional Theory, 2nd ed.; Wiley-VCH: New York, 2000. (37) De Jong, F.; Bour, J. J.; Schlebos, P. P. J. Inorg. Chim. Acta 1988, 154, 89–93.

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Table 5. Summary of the Pt-P Bond Distances

a

Mean value.

Table 6. Selected Bond Lengths (A˚) and Torsion Angles (deg) in Computed B3LYP/def2-SVP cis-[PtX2(PPh3)2] Compounds X

Pt-P1

Pt-P2

Pt-X1

Pt-X2

Aa

Bb

NO3 (5) AcO (3) Cl (1) Br (7) I (11) NO2 (14) Ph (22)

2.301 2.306 2.320 2.333 2.353 2.370 2.459

2.312 2.318 2.337 2.354 2.375 2.397 2.459

2.094 2.079 2.393 2.534 2.739 2.107 2.069

2.091 2.072 2.368 2.506 2.707 2.090 2.069

8.8 -4.7 -5.7 -5.1 -3.8 -13.8 29.7

-48.7 49.0 50.7 50.9 51.4 -38.8 30.1

b

a Lowest X2-Pt-P1-C torsion angle (phosphine A of Scheme 1). Lowest X1-Pt-P2-C torsion angle (phosphine B of Scheme 1).

Table 7. Selected Bond Lengths (A˚) and Torsion Angles (deg) in Computed B3LYP/def2-SVP cis-[PtClX(PPh3)2] Compounds X

Pt-P1

Pt-P2

Pt-O/N/C

Pt-Cl

Aa

Bb

NO3 (4) AcO (2) Cl (1) Br (6) I (10) NO2 (13) Ph (21)

2.322 2.318 2.320 2.319 2.318 2.314 2.293

2.311 2.319 2.337 2.352 2.374 2.396 2.515

2.092 2.072 2.368 2.503 2.699 2.076 2.052

2.383 2.388 2.393 2.394 2.398 2.393 2.399

-3.3 -2.7 -5.7 -2.7 -1.5 -11.1 11.2

52.4 48.7 50.7 50.7 50.8 -48.6 -48.3

b

a Lowest Cl-Pt-P2-C torsion angle (phosphine B of Scheme 1). Lowest X-Pt-P1-C torsion angle (phosphine A of Scheme 1).

considering the mixed ClX complexes: Ph > NO2 > I > Br > Cl > AcO g NO3. It is interesting to analyze the case of the ClPh complex (21), essentially the only one where the structural cis

influence is detectable and compare it with the Cl2 (1) and the Ph2 (22) complexes. Comparison with Cl2 indicates that the Pt-P1 bond length in ClPh is shortened by 0.027 A˚, showing the lower cis influence of Ph with respect to Cl. On the other hand, Pt-P2 is elongated by 0.178 A˚, due to the much higher trans influence of Ph with respect to Cl. Such elongation shows also the lower magnitude of the cis over the trans influence of Ph. Comparison with the Ph2 complex shows that Pt-P1 in ClPh is shortened by 0.166 A˚ and Pt-P2 is elongated by 0.056 A˚, due to the substitution of a Ph with a lower trans-influent and higher cis-influent Cl ligand. As a consequence of such different cis/trans influence of the two ligands, the Pt-P bond lengths in the ClPh complex present a very large asymmetry, differing by 0.222 A˚. Spectroscopic Cis and Trans Influences. Table 8 reports the 31P{1H} NMR spectra of some cis-[PtX2(PPh3)2] and cis-[PtXY(PPh3)2] complexes. In the case of the latter, assignment of the two resonances was made by comparison of their chemical shifts with those of the X2 derivatives. As a proof of correctness, in the case of cis-[PtCl(AcO)(PPh3)2] (2) we have performed a P-H HMBC (heteronuclear multibond correlation) experiment which showed a cross-peak between the resonance of the acetato protons and the P resonance at 3.1 ppm, in agreement with the assignment of this resonance to P trans to AcO. If we shall follow, again, a first approach considering only the X2 complexes, and neglecting, in the first approximation, the cis influence, or assuming that it has an irrelevant weight, the 1JPtP values of these complexes

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give the “apparent” order of trans influence (increasing coupling constants, decreasing trans influence): Ph > Me > SPh > SePh > NO2 > I > N3 > Br > Cl > AcO > CF3 COO > F > NO3 However, inspection of the data of the mixed XY complexes shows that the cis influence must not be neglected. For instance, let us consider cis-[PtCl(NO3)(PPh3)2] (4): while 1JPtP2 is higher than that of 1, in agreement with the fact that the nitrato ligand displays a trans influence lower than that of Cl, 1JPtP1 is also higher than that of 1, suggesting that the low cis influence8 of NO3 is operative. In our previous paper on trans-[PtXY(PPh3)2],8 we confirmed the already noticed6a,38,45 additivity of the contributions of the cis influences to 1JPtP. We tried and apply an additive scheme in this case to both cis and trans influences. The coupling constants 1JPtP of each P nucleus in compounds cis-[PtX2(PPh3)2] and cis-[PtXY(PPh3)2] can be considered as the sum of all the contributions given by the cis and trans ligands PPh3, X and/or Y to a basic value (N), considered as a constant starting point for this class of square planar, neutral Pt(II) compounds, and which represents the energy of the interaction between the nuclear spins of Pt and P. This value is perturbed by the presence of the ligands. If we call a, b, c, etc. the cis contributions and R, β, χ, etc. the trans contributions (Here, a, R for PPh3; b, β, for Cl; c, χ, for AcO; d, δ, for NO3; e, ε for Br; f, φ for I; g, γ for NO2; h, η for PhS; i, ι for F3CS; l, λ, Me; m, μ, Ph. See Table 8.), the Pt-P coupling constant of 1 is N þ a þ b þ β ¼ 3673 Hz

ð1Þ

1

the JPtP values of cis-[PtCl(NO3)(PPh3)2] (4) are N þ a þ d þ β ¼ 3842 Hz ðfor P1Þ

ð2Þ

N þ a þ b þ δ ¼ 3858 Hz ðfor P2Þ

ð3Þ

and

The contributions of Cl cannot be obtained with the present treatment, but they can be used as references, thus d and δ are the cis and trans contributions of NO3 relative to Cl. Performing similar calculations for all complexes, we obtain the contributions of the two influences of the various ligands. The equations considered and the results are reported in Table 8. Note that we obtain two similar but different values for each contribution, depending on the subtraction considered. For instance, for d, 1 - 2=-169 Hz, whereas 3 - 4= -155 Hz. These differences are too large to be only due to the reproducibility of the measure of the coupling constants ((3 Hz). We believe that these differences arise mainly from the fact that both subtractions reduce to b - d, eliminating N and especially a, whose constancy is a working hypothesis, useful for the present treatment. In particular, the value of N can be safely considered constant as explained above, while, on the contrary, the values of a (and also R) may depend on the overall coordination set and can vary according to the nature of the other ligands (X and Y). In fact, while d obtained from 1 - 2 is referred to the Cl2 complex, subtracting 3 - 4 gives d referred to the mixed Cl(NO3) complex. Thus the contributions of the phoshines, a, and of the ligand X/Y depend not only on their nature, but also on that of all their neighbors. The additivity scheme must therefore be applied with great care in the present case, where both the cis and trans influences are taken into account. Despite this drawback, however, some consistency is obtained if we refer to a homogeneous class of compounds, such as the ClX derivatives. In this case, the values of c and χ, d and δ, etc. discriminate between the cis and trans influences and give a sequence of both and, more importantly, an insight into their relative weights. Since we have subtracted 1 - 2, 3 - 4, etc., negative values mean higher contributions to 1JPtP of X compared to Cl and then a lower influence. Under the above-discussed approximations, we obtain the following series: cis f < e < b < i < d≈h < g < c < l, m

whereas the Pt-P coupling constant of both P nuclei of cis-[Pt(NO3)2(PPh3)2] (5) is N þ a þ d þ δ ¼ 4013 Hz

I > Br > Cl > SCF3 > NO3 ≈SPh > NO2 ≈AcO > Me > Ph

ð4Þ

Subtracting 1 - 2, or 3 - 4, will reduce to b - d, the difference between the cis contributions of NO3 and Cl. (38) Yamashita, F.; Kuniyasu, H.; Terao, J.; Kambe, N. Inorg. Chem. 2006, 45, 1399–1404. (39) Kirij, N. V.; Tyrra, W.; Naumann, D.; Pantenburg, I.; Yagupolskii, Yu. L. Z. Anorg. Allg. Chem. 2006, 632, 284–288. (40) Cobley, C. J.; Pringle, P. G. Inorg. Chim. Acta 1997, 265, 107–115. (41) Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy, K. G.; Prock, A.; Giering, W. P. Organometallics 1999, 18, 474–479. (42) Eaborn, C.; Odell, K. J.; Pidcock, A. J. Chem. Soc., Dalton Trans. 1978, 357–368. (43) Braterman, P. S.; Cross, R. J.; Young, G. B. J. Chem. Soc., Dalton Trans. 1977, 1892–1897. (44) Kreutzer, P. H.; Schorpp, K. T.; Beck, W. Z. Naturforsch. 1975, 30b, 544–549. (45) Additivity has been noted also for the kinetic cis effect, for example: Jaganyi, D.; Hofmann, A.; van Eldik, R. Angew. Chem., Int. Ed. 2001, 40, 1680–1683.

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trans λ, μ < η < γ < ι < β≈ε < φ < χ < δ Ph, Me > SPh > NO2 > SCF3 > AcO > Br≈Cl > I > NO3 Some points are worthy of discussion: (i) There is a span of approximately 2000 and 1000 Hz for the trans and cis influences, respectively. The latter, therefore, has a lower weight, but our data show that it is by no means irrelevant (in some cases it appears to be even higher). (ii) We confirm the almost opposite trend of the cis and trans influence series.4

132 Inorganic Chemistry, Vol. 49, No. 1, 2010 Table 8. 31P NMR Data for Some cis-[PtX2(PPh3)2] and cis-[PtXY(PPh3)2] Complexesa

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Table 8. Continued

a CDCl3 solutions, unless otherwise stated; δP values in parts per million versus aqueous H3PO4 (for compounds 21, 22, and 24, different references were used, and here, the δP values have been recalculated to agree with all other data); 2JPP are in the 12-20 Hz range, see the Experimental Section. b a, b, c, etc. are the contributions of the cis ligands; R, β, χ, etc. are the contributions of the trans ligands (a, R = PPh3; b, β = Cl; c, χ = AcO; d, δ = NO3; e, ε = Br; f, φ = I; g, γ = NO2; h, η = PhS; i, ι = F3CS; l, λ = Me; m, μ = Ph); and N is the basic value; such contributions are given mainly as the difference from Cl2. c C6D6. d CD2Cl2. e CH2Cl2. f C6H6.

(iii)

The cis influence series is similar to that obtained in the previous work,8 the positions of NO2 and AcO are inverted, but the contributions of these ligands are very similar in both cis and trans derivatives. (iv) The values of the cis contributions obtained here are about 30% higher than those calculated in the previous paper8 from the Pt-P coupling constants of the trans-[PtXY(PPh3)2] isomers, showing that the actual contribution of a given ligand depends on the system under study. (v) The trans influence series parallels that obtained above considering only the X2 complexes, with the notable exception of the position of AcO, which becomes higher in the series when the cis influence is taken into account, clear-cut evidence that the cis influence cannot be neglected. (vi) The trans sequence of the halides (Br g Cl g I) is unexpected, but their contributions are similar and low and can be affected by some uncertainty. The sequence I > Br > Cl seems well established in the literature,46 but such reports do not take into account the contribution of the cis influence. While the order (I > Br > Cl) of the kinetic trans effect is well-established,47,48 the fact that, on the contrary and according to our results, their trans influences are similar stresses the different significance and origin (ground state versus transition state stabilization) of the two phenomena.49

(46) See, for instance, Bennett, M. A.; Bhargava, S. K.; Priver, S. H.; Willis, A. C. Eur. J. Inorg. Chem. 2008, 3467–3481, and references therein. (47) Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 3rd ed.; John Wiley and Sons: New York, 1995. (48) The rather higher trans effect of I compared with Cl is the basis for a stereoselective synthesis of cisplatin (cis-[PtCl2(NH3)2]): [PtCl4]2- is first allowed to react with KI to give [PtI4]2- which, upon treatment with ammonia, gives cis-[PtI2(NH3)2] in very high isomeric purity. cis-[PtCl2(NH3)2] is then obtained by metathesis reactions. See: Dhara, S. C. Indian J. Chem. 1970, 8, 193–194. The direct reaction of [PtCl4]2- with ammonia gives cisplatin contaminated by the trans isomer. (49) To our knowledge there is at least one example in which the order of the trans influence is opposite to that of the trans effect, see: Wendt, O. F.; Elding, L. I. J. Chem. Soc., Dalton Trans. 1997, 4725–4731.

(vii)

The cis contributions of the halides are higher than their trans contributions. As argued in the previous paper,8 the cis influence of a ligand is related to the lowering of the positive charge on Pt induced by that ligand. The halides display a high cis influence because, being both σ and π donor,50 they decrease the positive charge on Pt efficaciously (as shown below), decreasing the P f Pt donation.51 (viii) There is no net parallelism between the structural and spectroscopic series, as a confirmation of the well-known dependence on the physical method used to establish such series.4,46 In order to check the general validity of the additivity of the contributions of the various X ligands calculated with respect to cis-[PtClX(PPh3)2] (Table 8), let us take into consideration a compound without Cl as a ligand, cis-[PtBr(AcO)(PPh3)2] (8). Referring to compounds 3 and 7, we obtain ε - χ=76 or 81 (confirming the higher trans influence of Br than AcO) and e - c=-293 or -288 (confirming the lower cis influence of Br than AcO). However, if the contributions are calculated using compound 1 as reference, we obtain ε - χ=(β - χ) - (β - ε)= (130) - (8)=122 Hz and e - c=(b - c) - (b - e)=(-266) - (55)=-321 Hz. Although the calculated values of χ and c reflect the general trend, they are rather different. The same discrepancies are obtained with the other compounds without Cl, 9 and 12. This fact clearly confirms that the actual contributions of the ligands depend from the nature of its neighbor Y, indicating that there is cooperation between the two cis ligands. Natural Bond Orbital (NBO) Analysis. To give a possible explanation of the different trans/cis influence of the (50) Bridgeman, A. J.; Gerloch, M. J. Chem. Soc., Dalton Trans. 1995, 197–204. (51) Strong donor ligands display also a strong (kinetic) cis effect because, lowering the positive charge on Pt, they reduce its electrophilicity. See: (a) Hofmann, A.; Dahlernburg, L.; van Eldik, R. Inorg. Chem. 2003, 42, 6528– 6538. (b) Jaganyi, D.; Reddy, D.; Gertenbach, J. A.; Hofmann, A.; van Eldik, R. Dalton Trans. 2004, 299–304.

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Table 9. Pt and P Natural Orbitals Involved in the Pt-P Bonds and Natural Charges on Pt, qPt (in e), in Computed B3LYP/def2-SVP cis-[PtX2(PPh3)2] Compounds bond orbital X

Pt-P1

Pt-P2

qPt

I (11) Br (7) Cl (1) AcO (3) NO3 (5) NO2 (14) Ph (22)

33.72%(sd1.06)Pt þ 66.28%(sp2.94)P1 33.51%(sd1.06)Pt þ 66.49%(sp2.93)P1 31.47%(sd1.08)Pt þ 68.53%(sp2.88)P1 32.44%(sd1.07)Pt þ 67.56%(sp3.01)P1 30.46%(sd1.12)Pt þ 69.54%(sp3.08)P1

32.98%(sd1.08)Pt þ 67.02%(sp2.96)P2 32.84%(sd1.10)Pt þ 67.16%(sp2.95)P2 30.82%(sd1.10)Pt þ 69.18%(sp2.91)P2 32.01%(sd1.11)Pt þ 67.99%(sp3.05)P2 29.45%(sd1.10)Pt þ 70.55%(sp3.10)P2

0.06 0.14 0.20 0.45 0.44 0.27 0.19

Table 10. Pt and P Natural Orbitals Involved in the Pt-P Bonds and Natural Charges on Pt, qPt (in e), in Computed B3LYP/def2-SVP cis-[PtClX(PPh3)2] Compounds bond orbital X

Pt-P1

I (10) Br (6) Cl (1) AcO (2) NO3 (4) NO2 (13) Ph (21)

33.13%(sd1.10)Pt þ 66.87%(sp2.83)P1 33.51%(sd1.10)Pt þ 66.49%(sp2.88)P1 33.51%(sd1.06)Pt þ 66.49%(sp2.93)P1 32.98%(sd1.00)Pt þ 67.02%(sp2.98)P1 33.24%(sd0.99)Pt þ 66.76%(sp3.06)P1 31.92%(sd0.85)Pt þ 68.08%(sp3.00)P1 31.71%(sd0.85)Pt þ 68.29%(sp2.69)P1

ligands, we have performed a natural bond orbital (NBO) analysis25 on the optimized structures discussed in a previous section. We were interested, in particular, to analyze the bonding orbitals used by Pt and P in the Pt-P bonds. One bond coupling constants are in fact supposed to be dominated by the Fermi contact term,52 which takes into account the s-character of the bonding orbitals and the electron densities of the ns valence orbitals at the nuclei. In the complexes studied in this paper, both the s-character of the hybrid orbitals and the electron density of the 3sp3 orbital of phosphorus and sd of platinum have been considered to be rather similar throughout,3,4,7b,7c,53 but we wanted to have a deeper look at these points, since these factors can be related to the different trans and cis influences of the ligands in the complexes examined. The results, reported in Tables 9 and 10 for cis-[PtX2(PPh3)2] and cis-[PtClX(PPh3)2] complexes, respectively, are far to be conclusive, also because in the cases of the long Pt-P bonds, the involved orbitals are interpreted by NBO analysis as lone pairs rather than bonding orbitals, so they have not been included in Tables 9 and 10. Some observations are, however, noteworthy: (i) For cis-[PtX2(PPh3)2] compounds (Table 9), the s-character of both P and Pt orbitals involved in the Pt-P bonds is almost the same. Averaging on Pt-P1 and Pt-P2 bonds, the hybridization degrees of sdx and spy orbitals vary from x=1.07 to 1.11 and from y=2.90 to 3.09, respectively, and in particular, they do not show any clear correlation with the structural and spectroscopic influences reported above. It therefore seems that for such complexes the s-character of Pt and P orbitals involved in the Pt-P bonds does not represent a relevant factor in explaining the trans influence of the ligands. (52) Jameson, C. In Multinuclear NMR; Mason, J., Ed.; Plenum Press: New York, 1987; pp 89-131. (53) For instance: (a) Allen, F. H.; Pidcock, A. J. Chem. Soc. (A) 1968, 2700–2704. (b) Allen, F. H.; Sze, S. N. J. Chem. Soc. (A) 1971, 2054–2056.

Pt-P2 33.06%(sd1.04)Pt þ 66.94%(sp3.00)P2 32.84%(sd1.10)Pt þ 67.16%(sp2.95)P2 31.56%(sd1.18)Pt þ 68.44%(sp2.88)P2 32.60%(sd1.19)Pt þ 67.40%(sp2.94)P2

(ii)

(iii)

qPt 0.13 0.17 0.20 0.34 0.32 0.25 0.21

In cis-[PtClX(PPh3)2] complexes (Table 10), on the other hand, the s-character of the Pt-P1 bond orbitals shows a clear trend, increasing in the order I = Br < Cl < AcO < NO3 < NO2 = Ph for the sdx orbitals of Pt and decreasing in the same order (with the exception of NO2 and Ph) for the spy orbitals of P. As a result, the total s-contribution to the Pt-P1 bond is nearly constant and equal to 33% for all ligands, except for NO2 (34%) and Ph (36%). The slight increase of the s-character of Pt-P1 for X = NO2 and Ph explains its shortening with respect to the value of the other complexes, as obtained from geometry optimization calculations. It is to be noted that the sequence of increasing s-character of the sdx orbitals of Pt parallels approximately the order of cis influence obtained by NMR measurements reported here and in the previous work8 (the order of AcO and NO3 is inverted): the greater the s-character of such orbitals, the greater the 1JPtP1 value. It therefore appears that the cis influence is principally related with the s-character of the Pt valence orbitals. It is also to be stressed that, while the cis influence cannot be completely revealed by looking at the Pt-P1 bond length of the optimized structures of the mixed complexes, it manifests itself clearly in the nature of the associated bonding orbitals. As for the Pt-P2 bond orbitals of cis-[PtClX(PPh3)2] complexes (Table 10), where the trans influence of the ligands comes into play, the s-character displays an opposite trend of the sdx orbitals, as it decreases in the order Br > Cl > AcO > NO3, while the spy orbitals on the P2 atom do not show a clear trend. As a result, the total s-contribution to the Pt-P2 bond shows only a slight decrease from Br (33%) to NO3 (32%). Such an outcome, as discussed in previous point i, confirms that other factors

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seem principally responsible of the trans influence of the ligands. Further information derived from the natural orbital analysis of electron density concerns the atomic charges, whose values for Pt, qPt, are also reported in Tables 9 and 10 for cis-[PtX2(PPh3)2] and cis-[PtClX(PPh3)2] complexes, respectively. These quantities, which reflect the total electron density associated with the Pt atom show an interesting trend, which is in good agreement with what argued on the basis of the NMR measurements. The lowest positive charges are in fact obtained with the halides, in the order I < Br < Cl, i.e. for the ligands which display the highest cis influences, while the largest qPt values correspond to the low cis-influencing ligands AcO and NO3. Low positive charges on Pt are also obtained in the complexes with NO2 and Ph, for which a larger covalence degree of the Pt-X bond can be expected. All these changes in qPt point out the relative importance of also the electrostatic contribution to the Pt-P interaction. In the case of halides and anionic ligands (NO3, AcO), such contribution cannot be neglected, while it appears less relevant for complexes with metal-ligand bonds with a higher degree of covalency (ligand=Ph, NO2). Conclusions We have shown that in compounds of the type cis-[PtXY(PPh3)2] the cis and trans influences cooperate in determining the experimental bond lengths and one bond coupling constants 1JPtP of the Pt-P bonds. We have also obtained an insight on the magnitude of both influences from the values of such constants and found that the weight of the cis influence is not irrelevant and must be taken into account in establishing the trans influence scale. A commonly accepted interpretation of the trans influence is that two ligands trans to each other compete for a same hybrid orbital, which spans across the metal atom. As for the cis influence, it has been rationalized assuming that, in Pt(II), Pt-P back-donation can be considered little relevant,54 therefore, the Pt-P bonds can be described mainly as donation from an sp3 orbital of P to an hybrid orbital of Pt. Under (54) In Pt(II) complexes, there is a neat correlation between 1JPtP and the donor power of aryl phosphines, suggesting that π back-donation to P is negligible, see ref 40.

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this approximation, the Pt-P bonds become weaker as the metal atom becomes less positively charged,8 a trend confirmed by natural bond orbital analysis on the B3LYP/def2SVP optimized structures. Such a computational study has also pointed out that the s-character of Pt and P hybrid orbitals involved in the Pt-P bonds can be related to the cis influence of the ligands, while it does not represent a relevant factor in explaining their trans influence. It is interesting to compare the results here presented with those reported by our group in the previous paper,8 where we have evaluated the cis influences of some anionic ligands analyzing the 1JPtP values of the trans isomers of some compounds of the present study. Although the cis influence series are similar, the magnitudes calculated from the cis isomers are about 30% higher than those obtained previously. Clearly these weights depend on the system under study; for instance in the X2 complexes, a ligand, say AcO, is cis to a phosphine in the trans isomer, but cis to another AcO in the cis derivative. A related point is worth noting. As stated above, the weights of both influences of a given ligand depend not only on its nature but also on its neighborhood; for instance, the weight of AcO in a mixed complex X(AcO) varies according to the ligand X, suggesting a synergism between the two cis ligands. One can speculate that, since these weights are in Hz, they describe how a ligand X perturbs (modifies) the energy of the interaction between the nuclear spins of Pt and P. Clearly such perturbation must depend not only on the nature of X but also on its environment, the charge on Pt, etc., ultimately on the overall coordination set. Despite this limitation, however, both cis and trans influences have been clearly detected, although precise quantitative evaluation of their magnitudes requires further studies, which we aim to undertake in the near future. Acknowledgment. This work has been supported by the Ministero dell’Universita e della Ricerca Scientifica e Tecnologica. Supporting Information Available: X-ray crystallographic data of compounds 2, 3 3 0.25H2O, 4 3 CHCl3, 5 3 CH2Cl2, 5 3 CHCl3, 7, 11 3 H2O, and 13 in CIF format and figures of the crystal structures of the compounds 2, 4 3 CHCl3, 5 3 CH2Cl2, 5 3 CHCl3, and 7. This material is available free of charge via the Internet at http://pubs.acs.org.